The spray painting of automobile bodies, truck parts, appliances and other industrial goods is typically carried out in enclosed areas called paint spray booths (PSBs). These booths act to contain solvent fumes and oversprayed paint and reduce the chances of dust contamination in order to protect the paint booth operators and the painted articles. These booths vary in size, but are somewhat basic in their design and operation. A typical booth would thus consist of a work area, back section with mist eliminators and a sump.
The articles to be painted generally pass through the work area while an air flow makes overspray contact either water in the sump or spray from a water curtain. The air is scrubbed with recirculated water at the water curtain, passes through mist eliminators and is removed by an exhaust fan.
Even though paint transfer efficiencies have increased through improved application technologies, roughly one-half of all paint sprayed does not reach its intended article. As a result, significant concentrations of paint buildup in the system and agglomeration can occur. When solventborne paints are used, the resultant mass is a sticky, tacky material which can plug mist eliminators, shower heads, and even recirculating pumps. When waterborne paints are employed, they will remain dispersed in the spray booth water. When present, waterborne paints will not present the same problems as untreated solventborne paint (i.e., tackiness, clumps, etc.). However, failure to remove waterborne paints results in increasing COD (carbon oxygen demand) levels, increasing suspended solids, and increasing levels of foam. All of these conditions decrease water clarity.
When water quality decreases, scrubbing efficiency decreases leading to potentially hazardous conditions of unchecked paint emissions being discharged into the atmosphere. Such conditions may also present severe safety hazards to paint spray booth operators. The paint solids that are collected in the water can form suspensions which remain tacky and create expensive separation and disposal problems.
It is therefore desirable to treat paint spray booth water systems so as to reduce or prevent, as much as possible, the agglomeration and deposition of oversprayed paint on critical paint spray booth operating pads, to render the resultant sludge (in solventborne systems) non-tacky and easily removable, and to provide a water quality such that it can be recycled for use in the system.
The paint employed in typical paint spray booths fall into two generic classes, waterborne and solventborne. Currently, solventborne paints are predominant. However, increased restrictions upon the levels of volatile organic compound emissions are forcing industrial applications to switch to waterborne paints.
The differences in the two paint classifications can be found in their formulations. Solventborne paint typically consists of organic solvents (such as xylene), resin binders, pigments and additives. In waterborne systems, water is substituted for the organic solvent. Thus, the resin binders, pigments, and additives must all be rendered water soluble or dispersible. Resins which are not water soluble can be stabilized with emulsifiers and cosolvents or be reformulated.
The behavior of a solventborne and a waterborne paint will differ when each becomes overspray in a wet paint spray booth. Waterborne paints will disperse in the aqueous medium, while solventborne paints agglomerate into a tacky mass and adhere to paint spray booth operating pads. Because of this behavior in water, in the past, the chemical process by which each paint type is treated differs. In order to remove waterborne paint from an aqueous medium, they should be considered as hydrophilic colloids which must be rendered hydrophobic. The stability of the compounds in water arises from the surface charge generated from carboxylic, aliphatic or aromatic hydroxyl groups on the polymer backbone. The process of destabilizing such solutions is termed coagulation. Flocculation follows in this treatment process whereby the destabilized particles are induced to come together, make contact and form large agglomerants.
Solventborne paints are hydrophobic and their treatment involves partially dispersing the paint particles in the aqueous medium via an anionic dispersant. The partially hydrophilic particles can then be treated to render them non-tacky. This process is referred to as detackification.